Infrared sensor element

ABSTRACT

An infrared sensor element according to the present invention includes a substrate, and a lower electrode layer, a pyroelectric layer, and an upper electrode layer sequentially formed on the substrate. The substrate has a linear thermal expansion coefficient higher than that of the pyroelectric layer, and the pyroelectric layer includes a polycrystalline body having an in-plane stress in a compressive direction. Thus, the infrared sensor element realizes the pyroelectric layer having a high orientation in a polarization axis direction, an excellent pyroelectric property.

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/455,315, filed Apr. 25, 2012, which is a continuation ofU.S. patent application Ser. No. 12/994,188, filed Nov. 23, 2010, whichis a U.S. National Phase Application of PCT International ApplicationPCT/JP2009/002883, filed Jun. 24, 2009, the entire disclosures of whichare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an infrared sensor element having anelectromechanical conversion function, and a method for manufacturingthe same.

BACKGROUND ART

An oxide dielectric thin film having a perovskite structure is expressedby means of the general formula ABO₃, and shows excellentferroelectricity, piezoelectricity, pyroelectricity, and electroopticalproperty. This attracts attention as an effective material for variouskinds of devices such as sensors and actuators, and a range of itsapplication is expected to be rapidly extended in the future. Since alead zirconate titanate (PZT: general formula is Pb(Zr_(x)Ti_(1-x))O₃(0<x<1)) thin film serving as perovskite oxides has highpyroelectricity, it is used as an infrared sensor element such as aninfrared sensor. The infrared sensor uses a pyroelectric effect offerroelectricity. A ferroelectric body has spontaneous polarizationinside, and generates positive and negative charges on its surface. Inits steady state in the air, the infrared sensor combines with a chargeof a molecule in the air to be in a neutral state. When thisferroelectric body is heated by applied infrared ray, an electric signalcan be extracted from the ferroelectric body based on an infrared rayamount.

Attempts have been tried to produce the PZT thin film by a vapor-phasegrowth method represented by a vapor deposition method, sputteringmethod (sputter method), or CVD method (Chemical Vapor Depositionmethod), or a liquid-phase growth method represented by a CSD method(Chemical Solution Deposition method), or hydrothermal synthesis method.Among them, the CSD method is easy to control a composition, and easy toproduce the thin film with high reproducibility. In addition, as afeature, the CSD method can be low in cost required for a productionfacility and can be mass-produced.

FIG. 9 is a cross-sectional view of a conventional ferroelectric thinfilm element. Referring to FIG. 9, thermally oxidized film 12 having afilm thickness of 2000 Å is formed on silicon substrate 11, and Ti film13 having a film thickness of 300 Å is formed on thermally oxidized film12 by the sputtering method, and Pt film 14 having a film thickness of2000 Å is also formed on Ti film 13 by the sputtering method, and thesefilms are used as a substrate.

Hereinafter, a method for producing the conventional ferroelectricelement will be described.

First, 0.1 mol of lead acetate is added to 1 mol of acetic acid andstirred at 100° C. in a nitrogen atmosphere for about 1 hour. Thissolution is combined with 36 ml of a solution provided by preparingtitanium isopropoxide (Ti(OCH(CH₃)₂)₄) to be 1 mol/L with2-methoxyethanol and 64 ml of a solution provided by preparing zirconiumisopropoxide (Zr(OCH(CH₃)₂)₄) to be 1 mol/L with 2-methoxyethanol. Thissolution is further stirred at 120° C. in a nitrogen atmosphere forabout 3 hours, and cooled down to room temperature and prepared to be0.5 mol/L with 2-methoxyethanol. Furthermore, 0.2 mol of water is addedto this solution and stirred for about 1 hour, and then diethanolamineis added thereto and this is used as a PZT precursor solution. Then,this precursor solution is dropped onto the substrate, and spin-coatedunder the condition that 350 rpm×3 seconds, and 5000 rpm×20 seconds, anddried gel is produced by a heat treatment at 100° C.×15 minutes. Then,an organic substance is thermally decomposed at 400° C.×60 minutes.These steps are repeated three times, whereby thin film 15 having a filmthickness of 2000 Å is obtained. Thin film 15 is subjected to a heattreatment with an infrared rapid thermal annealing (RTA) furnace to becrystallized, whereby PZT thin film 15 is obtained. The heat treatmentis performed at atmosphere pressure, in a 100% oxygen atmosphere, at anannealing temperature of 650° C. for an annealing time of 15 seconds.Upper electrode layer 16 is further formed on PZT thin film 15.

FIG. 10 is a view showing polarization-electric field (P-E) hysteresisloops of a conventional infrared sensor element. FIG. 10 shows P-Ehysteresis loops showing ferroelectricity before and after repeating 10⁸polarization inversions of PZT thin film 15 produced by the conventionalmethod. The conventional infrared sensor element is low in squareness Mof the P-E hysteresis loop, that is, low in ratio of saturationpolarization P_(s) This is because a pyroelectric layer is low incrystalline orientation.

Patent document 1 is known as background art document informationregarding this application of the present invention.

BACKGROUND ART DOCUMENT Patent Document

-   [Patent document 1] Japanese Unexamined Patent Publication No.    H8-157260

DISCLOSURE OF THE INVENTION

An infrared sensor element according to the present invention has asubstrate, and a lower electrode layer, a pyroelectric layer, and anupper electrode layer sequentially formed on the substrate. The lowerelectrode layer includes a conductive oxide crystalline body, and thesubstrate has a linear thermal expansion coefficient higher than that ofthe pyroelectric layer, and the pyroelectric layer includes apolycrystalline body having an in-plane stress in a compressivedirection.

Thus, according to the present invention, pyroelectric property can beimproved. This is because the pyroelectric layer is improved incrystalline orientation. More specifically, according to the presentinvention, since the lower electrode layer includes the conductive oxidecrystalline body, it is hardly affected by a composition of thesubstrate, and a lattice constant of its main oriented surface can beclose to a lattice constant of a main oriented surface of thepyroelectric layer.

In addition, since the substrate has the linear thermal expansioncoefficient higher than that of the pyroelectric layer, a compressivestress can be applied to the pyroelectric layer in a step of forming theinfrared sensor layer. As a result, orientation of the pyroelectriclayer is enhanced in a polarization axis direction, and pyroelectricproperty can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an infrared sensor element accordingto an embodiment 1 of the present invention.

FIG. 2 is a view showing a X-ray diffraction pattern of a (001)/(100)surface of a pyroelectric layer according to the embodiment 1 of thepresent invention.

FIG. 3 is a view showing a X-ray diffraction pattern of a (004)/(400)surface of the pyroelectric layer according to the embodiment 1 of thepresent invention.

FIG. 4 is a view showing a P-E hysteresis loop of the infrared sensorelement according to the embodiment 1 of the present invention.

FIG. 5 is a cross-sectional view of an infrared sensor element providedwith a highly conductive layer according to the embodiment 1 of thepresent invention.

FIG. 6 is a view showing P-E hysteresis loops of an infrared sensorelement according to an embodiment 2 of the present invention.

FIG. 7 is a view showing a P-E hysteresis loop of an infrared sensorelement according to an embodiment 3 of the present invention.

FIG. 8 is a cross-sectional view of an infrared sensor element accordingto an embodiment 4 of the present invention.

FIG. 9 is a cross-sectional view of a conventional infrared sensorelement.

FIG. 10 is a view showing P-E hysteresis loops of the conventionalinfrared sensor element.

PREFERRED EMBODIMENTS FOR CARRYING OUT OF THE INVENTION

Hereinafter, a description will be made of embodiments regarding aninfrared sensor element according to the present invention and a methodfor producing the same, with reference to the drawings.

Embodiment 1

[P1] FIG. 1 is a cross-sectional view of an infrared sensor elementaccording to an embodiment 1 of the present invention. Referring to FIG.1, the infrared sensor element includes substrate 1, and diffusionprevention layer 2, lower electrode layer 3, pyroelectric layer 4, andupper electrode layer 5 sequentially formed on substrate 1.

Substrate 1 can include a material having a linear thermal expansioncoefficient and fracture toughness higher than that of pyroelectriclayer 4. For example, substrate 1 can include various kinds of materialssuch as a stainless steel material or a metal material such as aluminumor magnesium.

Diffusion prevention layer 2 is provided to suppress mutual diffusionbetween components of substrate 1 and pyroelectric layer 4, andpreferably includes an oxide material having no crystal grain boundaryof silicon dioxide or titanium dioxide.

Lower electrode layer 3 includes a material mostly including lanthanumnickel oxide (LNO: Chemical formula is LaNiO₃). Lanthanum nickel oxidehas a R-3c space group, and has a perovskite structure distorted in theform of a rhombohedron (rhombohedral system: a0=5.461 Å (a0=ap), α=60°,pseudo-cubic system: a0=3.84 Å). LNO is an oxide having metallicelectric conductivity and has resistivity of 1×10⁻³ (Ω·cm, 300 K) andhas a feature of not causing metal-insulator transition even when atemperature is changed.

A material mostly including LNO includes a material in which a part ofnickel is replaced with other metal. For example, the material includesLaNiO₃—LaFeO₃ replaced with iron, LaNiO₃—LaAlO₃ replaced with aluminum,LaNiO₃—LaMnO₃ replaced with manganese, and LaNiO₃—LaCoO₃ replaced withcobalt.

Pyroelectric layer 4 includes (001)-oriented PZT having a rhombohedralsystem or a tetragonal system. A composition of PZT is a composition(Zr/Ti=53/47) in the vicinity of a boundary (Morphotropic phaseboundary) between the rhombohedral system and the tetragonal system. Inaddition, the Zr/Ti composition of pyroelectric layer 4 is not limitedto Zr/Ti=53/47, but may be Zr/Ti=30/70 to 70/30. In addition, thecomponent of pyroelectric layer 4 may only have to be a perovskite oxideferroelectric body mostly including PZT such as the one containing anadditive such as Sr, Nb, or Al, or may be PMN (lead magnesium niobate),or PZN (lead zinc niobate).

In addition, pyroelectric layer 4 is preferably provided such that asurface vertical to substrate 1 is preferentially oriented in apolarization axis, and a surface parallel to substrate 1 is randomlyoriented. Thus, its structure is small in elasticity, and high infracture toughness, and suitable for a device oscillating repeatedlysuch as an actuator.

Here, PZT of the tetragonal system used in the embodiment 1 is amaterial having a lattice constant such that a=b=4.036 Å, c=4.146 Å.Therefore, LNO of the pseudo-cubic structure having a lattice constantsuch that a=3.84 Å has good lattice matching with PZT. The latticematching means lattice consistency between a unit lattice of PZT and aunit lattice of the surface of LNO. In general, it is reported that whena certain kind of crystal surface is exposed to a surface, its crystallattice tries to match with a crystal lattice of a film formed thereon,and an epitaxial crystal nuclear is likely to be formed in a boundarybetween the substrate and the film.

In addition, when a difference in lattice constant between a mainorientated surface of pyroelectric layer 4 and a main orientated surfaceof lower electrode layer 3 is within ±10%, the orientation in a(001)/(100) direction of pyroelectric layer 4 can be enhanced. Thus, theepitaxial crystal nuclear can be formed in the boundary between thesubstrate and the film.

A polycrystalline film which is preferentially oriented in a (100)direction can be formed on the various kinds of substrates by producingLNO by the following method. Therefore, LNO functions not only as thelower electrode layer, but also as an orientation control layer ofpyroelectric layer 4. Thus, a (001) surface and a (100) surface of PZT(lattice constant: 4.036, c=4.146 Å) can be selectively formed so as tohave the good lattice matching with a (100) oriented LNO surface(lattice constant: 3.84 Å). In addition, since the LNO thin film oflower electrode layer 3 has a polycrystalline structure, a PZT thin filmformed thereon has also a polycrystalline structure.

When the CSD method is used in a step of producing pyroelectric layer 4,an annealing step is needed at the time of film formation. Since PZT iscrystallized and reoriented at high temperature, a compressive stress isleft at the time of cooling to room temperature due to a difference inthermal expansion coefficient from that of substrate 1. When it isassumed that SUS304 of austenitic stainless steel defined in JIS(Japanese Industrial Standard) is used as substrate 1, a linear thermalexpansion coefficient of SUS304 is 173×10⁻⁷/° C. Since a linear thermalexpansion coefficient of PZT is 79×10⁻⁷/° C., and SUS304 has the higherlinear thermal expansion coefficient, PZT has a residual compressivestress in the in-plane direction. According to the embodiment 1, thecompressive stress is left in pyroelectric layer 4, and when it isassumed that the side of upper electrode layer 5 is an upper surface andthe side of substrate 1 is a bottom surface, the infrared sensor elementhas a structure having a projection on the upper surface side.

Here, a description will be made of an X-ray diffraction pattern of theproduced infrared sensor element. FIG. 2 is a view showing an X-raydiffraction pattern of a (001)/(100) surface of the pyroelectric layerin the embodiment 1 of the present invention. FIG. 3 is a view showingan X-ray diffraction pattern of a (004)/(400) surface of thepyroelectric layer in the embodiment 1 of the present invention.

As can be seen from FIG. 2, pyroelectric layer 4 including PZT isselectively oriented only in the (001)/(100) direction of PZT. Inaddition, referring to FIG. 3, pyroelectric layer 4 has high selectiveorientation in a (004) direction serving as the polarization axisdirection.

Here, when it is defined that orientation (cc (004)) of a (004) surfaceis such that α (004)=I (004)/(I (004)+I (400)), a result is that α(004)=93%, which shows very excellent orientation in the (004)direction.

Thus, it can be found that orientation in a (001) direction serving asthe polarization axis direction can be enhanced by applying thecompressive stress to PZT. In addition, it is believed that when thecompressive stress is applied to pyroelectric layer 4, the (100)oriented crystal structure is deformed to the (001) orientatedstructure.

Upper electrode layer 5 is formed of gold (Au) having a thickness of 0.3μm. The material of upper electrode layer 5 is not limited to Au, and itonly has to be a conductive material, and the thickness may be within arange of 0.1 to 0.5 μm.

FIG. 4 is a view showing a P-E hysteresis loop of the infrared sensorelement in the embodiment 1 of the present invention. It is found thatthe P-E hysteresis loop is considerably excellent in squareness in FIG.4 as compared with the conventional example in FIG. 10.

According to the embodiment 1, when the squareness M=P_(r)/P_(s) iscalculated from the P-E hysteresis loop shown in FIG. 4, a result isthat M=0.78, so that an extremely preferable property is obtained. Thatis, a value of the remanent polarization Pr shows a high value such thatP_(r)=31.4 (μC/cm²).

The sensitivity of an infrared sensor element is linear to the value ofa pyroelectric coefficient. The pyroelectric coefficient is calculatedfrom a temperature dependency of the remanent polarization P_(r) and ahigh polarization value at a room temperature is preferable. Theinfrared sensor element of the embodiment 1 shows a very high value ofthe pyroelectric coefficient, 50 (nC/cm²/K).

Next, a description will be made of a method for producing the aboveinfrared sensor element.

First, a SiO₂ precursor solution is applied by a spin coating method toform diffusion prevention layer 2 on substrate 1. As the SiO₂ precursorsolution, while various kinds of solutions produced by the well-knownmethod can be used, Si—05S produced by KOJUNDO CHEMICAL LABORATORY CO.,LTD is used in the embodiment 1. This SiO₂ precursor solution is appliedonto substrate 1 by the spin coating method. The spin coating method isperformed at rotation speed of 2500 rpm for 30 seconds. The SiO₂precursor solution applied on substrate 1 is dried at a temperature of150° C. for 10 minutes, and then subjected to a primary baking treatmentat 500° C. for 10 minutes. Thus, the above steps are repeated until apredetermined film thickness is obtained, whereby diffusion preventionlayer 2 is formed.

Then, an LNO precursor solution is applied by the spin coating method toform lower electrode layer 3. The LNO precursor solution is applied ontodiffusion prevention layer 2 by the spin coating method at rotationspeed of 3500 rpm for 30 seconds.

This LNO precursor solution is prepared by a following method.

As a start raw material, lanthanum nitrate hexahydrate, and nickelacetate tetrahydrate are used, and 2-methoxyethanol and 2-aminoethanolare used as solvents. Since 2-methoxyethanol slightly contains water, itis used after its water is removed with 0.3 nm of molecular sieves.

First, lanthanum nitrate hexahydrate (La(NO₃)₃.6H₂O) is put into abeaker, and dried at 150° C. for one hour or more to remove hydrate.Then, it is cooled down to room temperature, and 2-methoxyethanol isadded and stirred at room temperature for 3 hours to dissolve lanthanumnitrate (solution A).

Meanwhile, nickel acetate tetrahydrate ((CH₃COO)₂Ni.4H₂O) is put into aseparate separable flask, and dried at 150° C. for 1 hour to removehydrate, and then dried at 200° C. for 1 hour, so as to be dried for 2hours in total. Then, 2-methoxyethanol and 2-aminoethanol are added andstirred at 110° C. for 30 minutes (solution B).

This solution B is cooled down to room temperature, and the solution Ais put into the separable flask containing the solution B. These mixtureis stirred at room temperature for 3 hours, whereby the LNO precursorsolution is produced.

Then, the LNO precursor solution is applied onto substrate 1 and driedat 150° C. for 10 minutes to perform a dewatering process. Then, aresidual organic component is thermally decomposed by a heat treatmentas provisional baking at 350° C. for 10 minutes, whereby an LNOprecursor thin film is produced. The step of dewatering the appliedprecursor solution is performed to remove physically-adsorbed water inthe LNO precursor solution, and the temperature is preferably higherthan 100° C. but lower than 200° C. Since the residual organic componentin the LNO precursor film starts to dissolve at 200° C. or higher, thetemperature is set to prevent the water from being left in the producedfilm. Meanwhile, the temperature in the step of producing the LNOprecursor thin film in the provisional baking step to dissolve theorganic substance is preferably 200° C. or more but lower than 500° C.Since crystallization of the dried LNO precursor film rapidly makesprogress at 500° C. or higher, the temperature is set to prevent theorganic substance from being left in the produced film.

The steps from the step of applying the LNO precursor solution ontodiffusion prevention layer 2 to the step of producing the LNO precursorthin film are repeated several times. At the moment the predeterminedfilm thickness is provided, the film is rapidly heated with an RTA(Rapid Thermal Annealing) furnace, to perform crystallization annealing.The crystallization annealing is performed at 700° C. for 5 minutes atheating-up rate of 200° C./min. The temperature of the crystallizationannealing is preferably 500° C. or higher but 750° C. or lower. Then, itis cooled down to room temperature. Lower electrode layer 3 is formedthrough the above steps, whereby LNO is highly-oriented in the (100)direction. In order to obtain lower electrode layer 3 having thepredetermined film thickness, the steps from the application step to thecrystallization step may be repeated instead of repeating the steps fromthe application step to the thermal decomposition step and thenperforming the crystallization step.

Then, a PZT precursor solution to form pyroelectric layer 4 is appliedonto lower electrode layer 3 by an existing application method such as aspin coating method or dip coating method. The PZT precursor solution isapplied on lower electrode layer 3 by the spin coating method atrotation speed of 2500 rpm for 30 seconds.

The PZT precursor solution is prepared by a following method. Ethanol tobe used in this preparation method is dehydrated ethanol which ispreviously subjected to a dewatering process, to prevent metal alkoxidefrom being hydrolyzed with contained water.

First, as a start raw material to prepare a Pb precursor solution, leadacetate (II) trihydrate (Pb(OCOCH₃)₂.3H₂O) is used. This is put into aseparable flask, and dried at 150° C. for 2 hours or more to removehydrate. Then, dehydrated ethanol is added and dissolved and refluxed at78° C. for 4 hours, whereby the Pb precursor solution is produced. As astart raw materials to prepare a Ti—Zr precursor solution, titaniumisopropoxide (Ti(OCH(CH₃)₂)₄) and zirconium normal propoxide(Zr(OCH₂CH₂CH₃)₄) are used. Both of them are put into separate separableflasks and dissolved by adding dehydrated ethanol, and refluxed at 78°C. for 4 hours, whereby the Ti—Zr precursor solution is produced. ATi/Zr ratio is weighed to implement that Ti/Zr=47/53 based on a molarratio. The Ti—Zr precursor solution is mixed with the Pb precursorsolution. At this time, the Pb component is increased by 20 mol % withrespect to a stoichiometric composition (Pb(Zr_(0.53), Ti_(0.47))O₃).This is performed to compensate for the shortfall due to volatilizationof a zinc component at the time of annealing. The mixture solution isrefluxed at 78° C. for 4 hours, and acetylacetone is added as astabilizer, by 0.5 mol equivalent with respect to a total amount ofmetal positive ions, and further refluxed at 78° C. for 1 hour, wherebythe PZT precursor solution is produced.

Next, the PZT precursor solution is applied onto lower electrode layer 3and dried at 115° C. for 10 minutes to perform a dewatering process.Then, a residual organic component is thermally decomposed by a heattreatment as a provisional baking at 350° C. for 10 minutes, whereby aPZT precursor thin film is produced.

The step of dewatering the applied PZT precursor solution is performedto remove physically-adsorbed water in the PZT precursor solution, andthe temperature is preferably higher than 100° C. but lower than 200° C.Since the residual organic component in the PZT precursor film starts todissolve at 200° C. or higher, the temperature is set to prevent thewater from being left in the produced film. Meanwhile, the temperaturein the step of producing the PZT precursor thin film in the provisionalbaking step to dissolve the organic substance is preferably 200° C. ormore but lower than 500° C. This is because crystallization of the driedPZT precursor film rapidly makes progress at 500° C. or higher, and thetemperature is set to prevent the organic component from being left inthe produced film.

The steps from the step of applying the PZT precursor solution ontolower electrode layer 3 to the step of producing the PZT precursor thinfilm are repeated several times. At the moment the predetermined filmthickness is provided, the film is rapidly heated with a RTA (RapidThermal Annealing) furnace using a lamp heater, to performcrystallization annealing. The crystallization annealing is performed at650° C. for 5 minutes at heating-up rate of 200° C./min. The temperatureof the crystallization annealing is preferably 500° C. or higher but750° C. or lower. When the temperature is 750° C. or higher, Pbcontained in the film is insufficient because the Pb evaporates at thetime of forming the PZT film, so that a crystalline property is lowered.After the crystallization annealing step, it is cooled down to roomtemperature, whereby pyroelectric layer 4 is formed. In order to obtainpyroelectric layer 4 having the predetermined film thickness, steps fromthe application step to the crystallization step may be repeated insteadof repeating the steps from the application step to the thermaldecomposition step and performing the crystallization step. Through theabove steps, the PZT thin film highly oriented in the (001) directioncan be provided.

Finally, upper electrode layer 5 including Au is formed on pyroelectriclayer 4 by an ion beam deposition method. The method for forming upperelectrode layer 5 is not limited to the ion beam deposition method, anda resistance heating deposition method, or a sputtering method may beused.

According to the embodiment 1, since pyroelectric layer 4 including PZTis formed on lower electrode layer 3 including LNO, considerably highcrystalline orientation can be obtained as compared with the case whereit is formed on the Pt electrode like in the conventional infraredsensor element.

FIG. 5 is a cross-sectional view of an infrared sensor element having ahighly conductive layer in the embodiment 1 of the present invention.When the infrared sensor element is used in a device having electricconductivity further higher than that of LNO, highly conductive layer 6may be formed between lower electrode layer 3 and diffusion preventionlayer 2 as shown in FIG. 5. Highly conductive layer 6 is preferablyformed of a noble metal material or a noble metal oxide such asplatinum, ruthenium, iridium, rhenium, ruthenium oxide, iridium oxide,or rhenium oxide.

In addition, according to the embodiment 1, since diffusion preventionlayer 2, lower electrode layer 3, and pyroelectric layer 4 are formed bythe CSD method, there is no need to perform a vacuum process which isneeded in a vapor phase growth method such as the sputtering method, sothat a cost can be cut. Since LNO used in lower electrode layer 3 can beself-oriented in the (100) direction by being produced through the aboveproduction method, the orientation direction is not likely to depend onthe material of substrate 1. Therefore, the material of substrate 1 isnot limited. Thus, according to the embodiment 1, a certain material canbe selected for substrate 1, in view of a thermal expansion coefficientand fracture toughness.

Furthermore, according to the embodiment 1, since substrate 1 caninclude various kinds of materials such as the stainless steel materialhaving the high fracture toughness, its reliability can be enhanced ascompared with the conventional case using the Si substrate which is abrittle material. Therefore, the infrared sensor element according tothe present invention is suitable for a device which repeatsoscillation, such as a sensor or an actuator. When a defect such as afine crack is generated in the silicon substrate in the production step,the substrate could be fractured from that point. As compared with this,the material having high fracture toughness according to the embodiment1 can be considerably prevented from being fractured, so that aproduction yield of the device can be improved. Furthermore, thestainless steel material is very inexpensive as compared with the Sisubstrate, so that a substrate cost can be reduced to about 1/10.

In addition, according to the embodiment 1, since the polycrystallinematerial is used for pyroelectric layer 4, fracture resistance againstthe oscillation can be improved as compared with the case using a singlecrystalline material. This is because a bonding force in the in-planedirection of the substrate is strong in the case of the singlecrystalline material, so that the stress due to the oscillation cannotbe relaxed and the substrate is likely to be fractured, but in the caseof the polycrystalline material, since a grain boundary exists in thein-plane direction, the stress can be relaxed.

In addition, while the RTA furnace is used in the crystallizationannealing step in the embodiment 1, substrate 1 can be prevented frombeing oxidized by setting a heating atmosphere to an inert atmosphere.In addition, when a halogen lamp used in the RTA furnace is formed onlyon the surface side of pyroelectric layer 4, the heat is only appliedfrom the surface side of pyroelectric layer 4, and substrate 1 isprevented from being heated, so that substrate 1 can be prevented frombeing oxidized.

Furthermore, the heating furnace for the crystallization annealing isnot limited to the RTA furnace, and a laser annealing may be used. Whenthe laser annealing is used, it is desirable that substrate 1 issufficiently heated when a compressive stress is applied to pyroelectriclayer 4, and an atmosphere furnace is used together.

In addition, while pyroelectric layer 4 is formed by the CSD method inthe embodiment 1, but the method is not limited thereto, various kindsof film forming methods such as an aerosol deposition method (ADmethod), sputtering method, CVD method may be used instead.

When the AD method is used, pyroelectric layer 4 can be produced suchthat the high crystalline orientation is realized, and the compressivestress is applied thereto by forming an oxide film including thecomposition of pyroelectric layer 4 on lower electrode layer 3 in a filmforming chamber at room temperature, and then heating the resultant inthe same manner as the crystallization annealing performed by the CSDmethod. When the sputtering method or CVD method is used, pyroelectriclayer 4 can be produced such that the high crystalline orientation isrealized, and the compressive stress is applied thereto by previouslyheating substrate 1 when a film is formed in a chamber.

In addition, while diffusion prevention layer 2 is formed in theembodiment 1, diffusion prevention layer 3 may not be used when elementdiffusion from substrate 1 or pyroelectric layer 4 is not generated inthe production step, and lower electrode layer 3 may be directly formedon substrate 1.

Furthermore, while LNO is used for lower electrode layer 3 in theembodiment 1, the material is not limited thereto and various conductiveoxide crystalline bodies may be used instead of that material, andespecially a perovskite conductive oxide is desirable. For example, theperovskite oxide may be the one mostly including strontium ruthenate orlanthanum-strontium-cobalt oxide which is preferentially oriented in a(100) direction in a pseudo-cubic system. In these cases also, bysetting a lattice constant of a main oriented surface to be within ±10%of a lattice constant of a main orientated surface of pyroelectric layer4, the orientation in the (001)/(100) direction of pyroelectric layer 4can be enhanced. Thus, by applying the compressive stress topyroelectric layer 4, the orientation in the (001) direction which isthe polarization axis direction of pyroelectric layer 4 can be enhanced.

Embodiment 2

Hereinafter, an infrared sensor element according to an embodiment 2 ofthe present invention will be described with reference to the drawings.A main configuration of the embodiment 2 is the same as that of theembodiment 1. As for the same configuration as that of the firstembodiment 1, its description is omitted and a different point will bedescribed.

According to the embodiment 2, SUS430 of ferritic stainless steeldefined in JIS is used as substrate 1 on which an infrared sensorelement is formed. While a linear thermal expansion coefficient ofSUS430 is 105×10⁻⁷/° C., a linear thermal expansion coefficient of PZTserving as pyroelectric layer 4 is 79×10⁻⁷/° C. Since SUS430 has thehigher linear thermal expansion coefficient, PZT has a residualcompressive stress. However, since the linear thermal expansioncoefficient is smaller than that of SUS304, it is believed that thecompressive stress to PZT is smaller than the case where SUS304substrate is used.

FIG. 6 is a view showing P-E hysteresis loops of the infrared sensorelement according to the embodiment 2 of the present invention.Pyroelectric layer 4 includes (001) oriented PZT having a rhombohedralsystem or a tetragonal system. In FIG. 6, PZT has three differentcompositions (Zr/Ti=53/47, 60/40, and 65/35) in the vicinity of aboundary (Morphotropic phase boundary) between the rhombohedral systemand the tetragonal system.

As can be clear from FIG. 6, preferable hysteresis curves having lessleakage current are provided. A maximum polarization values at this time(Pmax: polarization values at the time of 400 kV/cm) are 31 μC/cm²(53/47), 40 μC/cm² (60/40), and 33 μC/cm² (65/35). Their relativedielectric constants are 430 (53/47), 620 (60/40), and 590 (65/35).Thus, it is found that especially when the Zr/Ti ratio is 60/40, themaximum polarizability and the relative dielectric constant are high andthus the dielectric/ferroelectric property is preferable.

In addition, the Zr/Ti compositions in pyroelectric layer 4 are notlimited to such that Zr/Ti=53/47, 60/40, and 65/35, it may be such thatZr/Ti=30/70 to 70/30. Furthermore, the Zr/Ti compositions is preferablethat Zr/Ti=50/50 to 70/30.

Embodiment 3

Hereinafter, an infrared sensor element according to an embodiment 3 ofthe present invention will be described with reference to the drawings.A main configuration of the embodiment 3 is the same as that of theembodiment 1. As for the same configuration as that of the firstembodiment 1, its description is omitted and a different point will bedescribed.

According to the embodiment 3, SUS304 is used for substrate 1, and asurface configuration of substrate 1 is a rough surface.

Here, warpages of the substrate before film formation, after anannealing treatment before the film formation, and after the filmformation are compared by used of a curvature radius R of the substrate.The fact that an absolute amount of the curvature radius is large meansthat a warpage amount is small. Meanwhile, the fact that the absoluteamount of the curvature radius is small means that a warpage amount islarge.

A working example uses a SUS304 substrate as substrate 1 having athickness of 0.2 mm, and having a rough surface whose surface roughness(Ra) is 93 nm. In addition, a comparison example uses a SUS304 substrateas substrate 1 having a thickness of 0.2 nm, and having a mirror surfacewhose surface roughness (Ra) is 30 nm. Table 1 shows a result ofperformance comparison between the working example and the comparisonexample. The result is defined assuming that the curvature radius of thesubstrate before the film formation is infinite (∞).

TABLE 1 SAMPLE Ra (nm) STEP R (mm) WORKING 93 BEFORE FILM FORMATION ∞EXAMPLE ANNEALING PROCESS 25318 (ROUGH TO SUBSTRATE ON SURFACE) WHICHFILM IS t = 0.2 mm NOT FORMED AFTER FILM FORMATION −14054 COMPARISON 30BEFORE FILM FORMATION ∞ EXAMPLE ANNEALING PROCESS 45383 (MIRROR TOSUBSTRATE ON SURFACE) WHICH FILM IS t = 0.2 mm NOT FORMED AFTER FILMFORMATION 139

Based on the result in the table 1, when the substrate on which the filmis not formed is only subjected to the annealing treatment, thecurvature radiuses are very large in both of the working example and thecomparison example. That is, it can be found that it is highly unlikelythat the substrate is warped due to the annealing treatment. Meanwhile,when the substrate on which the film is formed is measured, it is foundthat the curvature radius is large in the working example while thecurvature radius is small in the comparison example. That is, a warpageamount can be reduced in the case where the film is formed on thesubstrate having the rough surface, as compared with the case where thefilm is formed on the substrate having the mirror surface.

FIG. 7 is a view showing a P-E hysteresis loop of the infrared sensorelement according to the embodiment 3 of the present invention. Inaddition, a Zr/Ti composition in pyroelectric layer 4 is such thatZr/Ti=53/47. As can be seen from FIG. 7, even when the surface ofsubstrate 1 is the rough surface, a preferable hysteresis curve can beobtained.

Embodiment 4

Hereinafter, an infrared sensor element according to an embodiment 4 ofthe present invention will be described with reference to the drawings.As for the same configuration as that of the first embodiment 1, itsdescription is omitted and a different point will be described.

According to the embodiment 4, pyroelectric layer 104 is further formedon a second surface opposed to a first surface across substrate 1 onwhich lower electrode layer 3 and pyroelectric layer 4 are formed.

FIG. 8 is a cross-sectional view of the infrared sensor elementaccording to the embodiment 4 of the present invention. Referring toFIG. 8, the infrared sensor element is provided such that diffusionprevention layer 2, lower electrode layer 3, pyroelectric layer 4, andupper electrode layer 5 are sequentially formed on first surface 200 ofsubstrate 1. Furthermore, diffusion prevention layer 102, lowerelectrode layer 103, and pyroelectric layer 104 are sequentially formedon second surface 201 opposed to first surface 200 across substrate 1.

Substrate 1 can include a material having a linear thermal expansioncoefficient and fracture toughness higher than that of pyroelectriclayers 4 and 104. There are various kinds of materials such as astainless steel material, or a metal material such as aluminum ormagnesium.

Diffusion prevention layers 2 and 102 are provided to prevent mutualdiffusion of components between substrate 1 and pyroelectric layers 4and 104, and they preferably include an oxide material having no crystalgrain boundary of silicon dioxide or titanium dioxide.

Lower electrode layers 3 and 103 include a material mostly includingLNO. LNO has a perovskite structure. Pyroelectric layers 4 and 104 arepolycrystalline bodies each having an in-plane compressive stress.

Steps of forming diffusion prevention layer 102, lower electrode layer103, and pyroelectric layer 104 on second surface 201 are performedusing steps of forming the same layers on first surface 200. Forexample, the step of forming pyroelectric layer 104 includes a step ofapplying a precursor solution on the substrate, an annealing step ofcrystallizing the precursor solution under the condition that thesubstrate is heated, and a cooling step.

When the steps are the same on first surface 200 and second surface 201,there is a feature of being capable of performing the steps under thesame condition at the same time, basically. Thus, working efficiency canbe improved in the steps of forming the layers.

Regarding the infrared sensor element produced as described above,pyroelectric layer 4 has an in-plane compressive stress, and crystallineorientation is improved. As a result, pyroelectric property isexcellent.

Furthermore, when pyroelectric layer 104 is provided on second surface201 opposed to first surface 200, compressive stresses of twopyroelectric layers 4 and 104 are balanced to prevent warpage of theinfrared sensor element.

In addition, when pyroelectric layer 4 on first surface 200 andpyroelectric layer 104 on second surface 201 are provided so as to havethe same composition and the same film thickness, warpage is hardlygenerated in a center surface of substrate 1, and the infrared sensorelement is prevented from being warped.

In addition, the description has been made of the case where diffusionprevention layer 102, lower electrode layer 103, and pyroelectric layer104 are formed on second surface 201 of substrate 1 in the embodiment 4.However, when at least pyroelectric layer 104 is formed on secondsurface 201, it has an effect of balancing the compressive stress withpyroelectric layer 4 on first surface 200.

INDUSTRIAL APPLICABILITY

According to an infrared sensor element in the present invention, theinfrared sensor element show an excellent pyroelectric property.Therefore, the infrared sensor element according to the presentinvention can be effectively used for various kinds of sensors such asan angular velocity sensor, and a piezoelectric sensor, various kinds ofactuators such as a piezoelectric actuator, and an ultrasonic motor, andoptical devices such as an optical switch and an optical scanner, usedin various kinds of electronic equipments.

REFERENCE MARKS IN THE DRAWINGS

-   1 SUBSTRATE-   2 DIFFUSION PREVENTION LAYER-   3 LOWER ELECTRODE LAYER-   4 PYROELECTRIC LAYER-   5 UPPER ELECTRODE LAYER-   6 HIGHLY CONDUCTIVE LAYER-   11 SILICON SUBSTRATE-   12 THERMALLY OXIDIZED FILM-   13 TI FILM-   14 PT THIN FILM-   15 PZT THIN FILM

1. An infrared sensor element comprising: a substrate; a lower electrodelayer formed on the substrate; a pyroelectric layer formed on the lowerelectrode layer; and an upper electrode layer formed on the pyroelectriclayer, wherein the substrate has a linear thermal expansion coefficienthigher than that of the pyroelectric layer.
 2. The infrared sensorelement according to claim 1, wherein the pyroelectric layer is apolycrystalline body having an in-plane compressive stress.
 3. Theinfrared sensor element according to claim 1, wherein the lowerelectrode layer includes a conductive oxide crystalline body,
 4. Theinfrared sensor element according to claim 1, wherein the pyroelectriclayer is formed by a chemical solution deposition method.
 5. Theinfrared sensor element according to claim 1, wherein the substrate hasfracture toughness higher than that of the pyroelectric layer.
 6. Theinfrared sensor element according to claim 1, wherein the lowerelectrode layer is a perovskite type.
 7. The infrared sensor elementaccording to claim 1, wherein a difference between a lattice constant ofa main oriented surface of the lower electrode layer and a latticeconstant of a main oriented surface of the pyroelectric layer is within±10%.
 8. The infrared sensor element according to claim 1, wherein thepyroelectric layer includes a perovskite oxide ferroelectric bodyprovided such that a surface vertical to the substrate is preferentiallyoriented in a polarization axis direction, and a surface parallel to thesubstrate is randomly oriented.
 9. The infrared sensor element accordingto claim 1, wherein the substrate includes a metal material.
 10. Theinfrared sensor element according to claim 1, wherein a highlyconductive layer having resistivity lower than that of the lowerelectrode layer is formed under the lower electrode layer.
 11. Theinfrared sensor element according to claim 1, wherein the substrate hasa rough surface.
 12. The infrared sensor element according to claim 1,wherein a surface state of the substrate has a surface roughness Ra of90 nm or more.
 13. The infrared sensor element according to claim 1,wherein a pyroelectric layer is further formed on a second surfaceopposed to a first surface across the substrate having the lowerelectrode layer, and the pyroelectric layer the substrate has a linearthermal expansion coefficient higher than that of the pyroelectric layeron the second surface, and the pyroelectric layer on the second surfaceincludes a polycrystalline body having an in-plane stress.
 14. Theinfrared sensor element according to claim 13, wherein the pyroelectriclayer on the second surface is formed on the second surface through thelower electrode layer.